The problem with those products is that even if the key is changed (and the data can't be decrypted with the new key, thus, destroyed), there is always a possibility that the previous keys are kept in the chip.
So, let's say that you put some data on it delete it, and Mr. Joe get it.
- Mr. Joe - Hello Manufacturer, I have "accidentally erased my disk". Is there something I can do about it?
- Manufacturer - Ah, this brand of disk... No! It's designed to be non-recoverable...
And now, let's say that you put some really bad data on it, delete it, and NSA get it.
- NSA - Hello Manufacturer, I have this disk that is "erased". Is there something I can do about it?
- Manufacturer - Here's a software. Run it and tell me the disk serial #.
- NSA - Ok. Disk is S/N: SN12345
- Manufacturer - (Punching some numbers...) Ok, here is the backdoor key for this disk. You can use the software to revert to any of the previous encryption keys used in that specific disk.
The same hold true for "secure" flash memory, "secure" usb thumb drives, etc.
The point is either: You need to trust the manufacturer... Or you don't do anything bad that can get you in the hand of the FBI, NSA, etc with those devices :-) Do the second option, never rely on the first one!